1. Field of the Invention
The present invention relates to a dual range infinitely variable transmission and, more particularly, to a control strategy for an infinitely variable transmission which includes two power paths to a pair of planetary gearsets. The transmission utilizes a continuously variable speed mechanism in conjunction with a fixed ratio speed mechanism for the two power paths. A two-position jaw clutch means is utilized to switch between the single path high range and the dual path low range of the transmission. A reaction type clutch is utilized to lock the sun gear of the second planetary gear set to provide a positive neutral.
2. Description of the Prior Art
The typical continuously variable transmission (CVT) employs a continuously variable speed mechanism in the form of a variable pulley with a pair of flanges mounted on an input shaft such that at least one of the flanges is movable axially with respect to the other. A similar variable pulley is mounted on an output shaft. A flexible belt couples the pulleys to allow the transfer of torque between the shafts when one of the shafts is driven. When the pitch radius of one pulley is changed, the pitch radius of the other pulley is changed simultaneously in the opposite direction. As a result, the drive ratio between the input and output shafts is varied in a continuous, smooth manner within the ratio range of the speed mechanism.
Heretofore, efforts have been made to extend the range of the transmission drive ratio of a continuously variable transmission (CVT) beyond the ratio range of the variable speed mechanism, that is, to devise an infinitely variable transmission. One such effort utilized two power paths between the torque input source and the rotating output. One power path comprised the conventional variable speed mechanism described above with variable pulleys on the input and output shafts and a belt connecting the pulleys. The second power path comprised a pair of sprockets, one located on each of the input and output shafts, and a silent chain drivingly connecting the two sprockets. The silent chain drive provided a fixed ratio drive.
In general, an engine's output power is affected by the speed at which it is cycling. This relation between engine speed and output power is particularly evident in the gas turbine engine whose output power displays an approximate linear relation to the speed at which the turbine spins. An infinitely variable transmission (IVT) allows an engine to maintain a constant speed, and thus constant power output, as a vehicle's speed changes. As the speed of the vehicle changes, the transmission ratio adjusts to maintain essentially a constant engine speed and output power.
In light of the advantages proVided by a dual range infinitely variable transmission, and more particularly the benefits in using an IVT in conjunction with a gas turbine engine, it is desirable to have a control strategy for simultaneously accelerating a vehicle in response to a driver's demand and accelerating the engine in order to obtain the desired output power. At least two factors influence the determination of how to allocate the limited available engine power between accelerating the vehicle and accelerating the engine. First, it takes a finite period of time to increase the engine speed to a desired higher power speed due to the rotational inertia of the spinning engine parts. This problem is particularly evident in gas turbine engines whose rotating parts exhibit large moments of inertia. Furthermore, this period increases as more engine power is used for the purpose of accelerating the vehicle. Thus, allocating power to accelerating the vehicle decreases the rate at which higher power output is reached. Second, higher power output is not desirable if the torque supplied for turning the wheels exceeds the traction capabilities of the wheels and causes the wheels to lose traction with the driving surface. It is therefore desirable for the infinitely variable transmission to be controlled in such a manner as to balance the above factors in order to obtain a desirable acceleration rate.